In order to make high data rate interactive services such as video conferencing and internet access available to more residential and small business customers, high speed data communication paths are required. Although fiber optic cable is the preferred transmission media for such high data rate services, it is not readily available in existing communications networks, and the expense of installing fiber optic cable is prohibitive. Current telephone wiring connections, which consist of twisted pair media, were not designed to support the high data rates required for interactive services such as video on demand or even high speed interconnects. In response, Asymmetrical Digital Subscriber Line (ADSL) technology has been developed to increase the transmission capabilities within the fixed bandwidth of existing twisted pair connections, allowing interactive services to be provided without requiring the installation of new fiber optic cable.
Discrete Multi-Tone (DMT) is a type of ADSL technology that uses a multi-carrier technique that divides the available bandwidth of a communications channel such as a twisted pair connection into a number of frequency sub-channels. These sub-channels are also referred to as frequency bins or carriers. A specific DMT technique has been adopted by the ANSI T1E1.4 (ADSL) committee. For purposes of discussion, the term "DMT" within this specification, will generally refer to the standard. DMT is used to generate 250 separate 4.3125 kHz sub-channels from 26 kHz to 1.1 MHz for downstream transmission to the end user, and 25 sub-channels from 26 kHz to 138 kHz for upstream transmission by the end user. Each bin is allocated a number of bits to send with each transmission. The number of bits allocated per bin in a DMT system are 0, and 2-15 bits.
Prior to transmitting real-time data with a DMT system, an initialization process occurs. During a first portion of the initialization process, an activation and acknowledgment step occurs. It is during this step that a transmit activation tone is generated following power-up of the ADSL system. Transceiver training is the next step of the initialization process. During transceiver training, the equalization filters of the ADSL system are trained and system synchronization is achieved. Next, channel analysis and exchange are performed as part of the initialization processes. During the channel analysis and exchange, the signal to noise ratio of the channels is determined, and bit loading configuration information for the bins, and other configuration information, is transferred between transceivers.
Subsequent to the initialization process, real-time data transmission begins. During real-time data transmission, the DMT standard specifies that a fixed amount of power be transmitted on each carrier. However, a disadvantage to assigning a fixed amount of power is that when there are unneeded carriers due to a low data rate, there is unnecessary power consumption associated with assigning a fixed amount of power to a carrier that is not transmitting any data. This additional power results in additional system costs. Another disadvantage is that a carrier's signal is attenuated as it is transmitted over greater distances. As a result, when the signal for a given carrier is attenuated to the point where data cannot be transmitted with a desired certainty, the capacity of the bin is set to zero, while its transmit power remains allocated to the now unused bin. This power on unused bins is an inefficient use of system power. Therefore, it would be beneficial to optimize power based upon the number of used bins, or optimizing a data rate for a fixed amount of power.